1. Field of the Invention
The present invention relates to a printing apparatus, and, more particularly, to a printing apparatus in which a variation in a line feed length and a change in the physical properties of paper caused by a change in the temperature of the printing apparatus are compensated for, thereby improving the quality of printing and enabling paper to be easily picked up from a paper tray.
2. Description of the Related Art
A printing apparatus is an apparatus that prints an image or text on the surface of paper. FIG. 1 schematically illustrates the structure of a conventional printing apparatus. Referring to FIG. 1, the printing apparatus includes a paper feeding unit, a paper transferring unit, a printing unit, and a paper releasing unit. The paper feeding unit feeds paper P into the printing apparatus, and includes a paper tray 10 on which the paper P is placed and a pickup roller 12 that picks up the paper P placed on the paper tray 10. The paper transferring unit transfers the paper P supplied from the feeding unit to the printing unit, and includes a feed roller 14 that precisely transfers the paper P to the printing unit and a pressure roller 16 that applies a friction force to the feed roller 14 to aid in transferring the paper P. The printing unit prints an image or text on the paper P transmitted by the paper transferring unit, and includes a print head 18 that discharges ink droplets onto the paper P through a nozzle 20, and a carriage 22 to which the print head 18 is attached. The carriage 22 includes a temperature sensor 24 that controls the temperature of the nozzle 20 of the print head 18. The paper releasing unit releases the paper P on which an image or text has been printed by the printing unit and includes an exit roller 26, and an exit wheel 28.
In a conventional printing apparatus having the above structure, a sheet of the paper P stacked on the paper tray 10 is picked up by the pickup roller 12 and positioned between the feed roller 14 and the pressure roller 16. Then, the paper P is gradually moved by a line feed length according to a predetermined rotation angle of the feed roller 14. Precision in the line feed length is expressed as a resolution that is used to verify the quality of printing. In general, the printing apparatus is manufactured to have a resolution in a line feed length of 600 dots per inch (dpi). If the resolution is 600 dpi, the distance between adjacent dots is about 42 μm.
To precisely transfer the paper P, the dimensions of the outer diameter of the feed roller 14 must be precisely controlled. Typically, the outer diameter of the feed roller 14 is determined based on an assumption that the printing apparatus operates at room temperature, for example, 22° C., and the line feed length is set based on the determined outer diameter of the feed roller 14. However, the ambient temperature in which the printing apparatus operates may change within a wide range, approximately from 5° C., to 40° C. A change in the ambient temperature results in a change in the outer diameter of the feed roller 14, that is, the feed roller 14 may expand or contract, and as a result, the line feed length also changes. Therefore, when using the conventional printing apparatus, a change in the ambient temperature may degrade the quality of printing. For example, an unexpected white or black line could appear on the paper P.
The distance between the paper P and the nozzle 20 of the print head 18 depends on a ratio of the linear velocity of the outer surface of the feed roller 14 to that of the outer surface of the exit roller 26. The quality of printing is greatly affected by the distance between the paper P and the nozzle 20. In general, the feed roller 14 and the exit roller 26 are manufactured using different materials and the dimensions of their outer diameters differ from one another. A change in the ambient temperature results in a change in the ratio of the linear velocity of the outer surface of the feed roller 14 to that of the outer surface of the exit roller 26. A change in this ratio deteriorates the quality of printing.
The amount of moisture contained in the paper P decreases at a low temperature, and a reduction in the amount of moisture increases the stiffness of the paper P. In this case, the intensity of a force required to pick up the paper P from the paper tray 10 must be increased. A paper pickup system of a conventional printing apparatus is set to appropriately operate at room temperature, and, therefore, it would be difficult to pick up paper from a paper tray when operating the paper pickup system at low temperatures.